The present invention relates to uninterruptible power supplies and in particular, to a power supply that can directly couple a utility line to a pair of output terminals without passing that power through an inverter.
One type of presently available uninterruptible power supply employs a battery directly charged by utility lines. The battery in turn continuously powers an inverter to produce an alternating current on its output. Therefore, when the utility lines fail, the battery, retaining its charge, continues to energize the inverter so that power is not interrupted. A known inverter drives a transformer with a pulse train having a varying duty cycle. This duty cycle varies sinusoidally to construct a sine wave power output.
Also known is an off-line inverter which is connected to the primary of a ferroresonant transformer in conjunction with the A.C. utility. This dual primary transformer normally receives its power input from either the A.C. utility or the inverter. The ferromagnetic transformer's inherent energy storage capability supplies the load energy during the finite time it takes to switch between the utility and inverter and vice versa.
Another type of known uninterruptible supply uses line thyristors which are rendered non-conductive through self-commutation. This self-commutation requires passage of approximately one half of a power cycle before the line thyristors cease conducting. Especially with inductive loads, current may continue to flow even though the utility power has failed and triggering stopped. Consequently this inverter cannot replace the utility power until the passage of considerable time: when the line thyristors have ceased conducting. This delay causes undesirable transients in the supply output. The circuits described in U.S. Pat. Nos. 3,293,446 and 3,344,282 of Baude are illustrative of known uninterruptible power supplies utilizing line thyristors.
Another disadvantage associated with some conventional uninterruptible power supplies is a power loss resulting from a double conversion of the form of electrical energy: transformation from alternating to direct and then from direct to alternating current. In the case of the dual primary unit, a single conversion with a high loss ferromagnetic transformer is required. In either case, efficiencies of over 90% are rare. Accordingly, there is a need for a simple highly efficient uninterruptible power supply with a fraction of the losses presently generated in the known systems.